1. Field of the Invention
The present invention relates to a phase-shift mask which, taking advantage of the interference action of its phase shifter, improves the resolution of transfer patterns, and to a phase-shift mask blank for it. In particular, the invention relates to a halftone phase-shift mask and to a blank for it.
2. Description of the Related Art
A phase-shift process is one ultra-resolution technique in photolithography, and, when compared with a process of using an ordinary photomask in the same exposure wavelength range, it increases the contrast of transferred images and enables fine pattern transfer over wavelength limits. In the phase-shift process, a phase-shift mask is used for transferring fine patterns.
A halftone phase-shift mask is a type of phase-shift mask. It is said that the transmittance of the phase shifter part (semi-transmissive part) of the halftone phase-shift mask is from about a few % to tens % of that of the non-pattern part (light-transmissive part) thereof and halftone phase-shift masks are effective in forming contact holes and insulation patterns. Halftone phase-shift masks have become widely used mainly for forming contact holes, as they are relatively easy to fabricate through patterning. The halftone phase-shift mask comprises a light-transmissive part, and a semi-transmissive halftone phase shifter part having a function of shifting the phase of light. The phase of light having passed through the two parts of the mask is shifted generally to 180° to cause mutual interference at the pattern boundaries, and the contrast of the transferred images is thereby increased. As a result, the focal depth to attain the necessary resolution of the mask is enlarged, and it is therefore possible to increase the resolution of the mask and to broaden the process latitude thereof without changing the exposure light wavelength. From the layer constitution of the halftone phase shifter part thereof, the halftone phase-shift mask is grouped into two types, a single-layered mask and a multi-layered mask.
For the single-layered phase-shift mask, for example, known are SiOx-based or SiOxNy-based masks such as those described in Japan Patent Laid Open Hei 7-199447, and SINx-based masks such as those described in Japan Patent Laid Open Hei 8-211591. Some examples of the multi-layered phase-shift mask are described in U.S. Pat. No. 5,939,227 and Japan Patent Unexamined Publication 2000-511301.
The advantage of the single-layered halftone phase-shift mask is that its structure is simple and its production is relatively easy. Therefore, most halftone phase-shift masks now produced on a large scale are single-layered ones. On the other hand, the advantage of the multi-layered halftone phase-shift mask is that different parameters of transmittance and phase shift degree that must be controlled in masks can be controlled independently therein.
In most multi-layered masks, the halftone phase shifter part comprises a combination of a light-shielding layer and a transparent layer. The light-shielding layer is made of a material capable of shielding the light that falls within an exposure wavelength range; and the transparent layer is made of a transparent material of which the transmittance within an exposure wavelength range is at least 80%. In such a multi-layered halftone phase-shift mask, the light-shielding layer acts to control the transmittance of the mask to a practical degree, and the transparent layer thereof acts to ensure the necessary phase shift to 180° relative to the pattern aperture (light-transmissive part). To that effect, the two layers in the multi-layered halftone phase-shift mask have different roles and functions independent of each other.
To meet the prospective requirement for finer circuit patterns, it is now inevitable to further shorten the wavelength of light for exposure even though the best use of the phase-shift ultra-resolution technique is made. At present, an ArF excimer laser of 193 nm and an F2 excimer laser of 157 nm are being investigated for the exposure light source in photolithography in the coming generation. However, there are known only a few transparent materials having a transmittance of at least 80% in such a exposure wavelength range, for example, CaF2 and high-purity quartz. Most materials absorb at least about 20% of light and reflect at least about 10% of light in that range. In other words, the materials except CaF2 and high-purity quartz, which are for the transparent layer in the current photomasks to be exposed to ordinary light longer than the short-wave light as above, could not form a transparent layer but form a high-transmission having a transmittance of smaller than 70% in the coming photomasks to be exposed to the short-wave light as above. Therefore, in case where the current multi-layered halftone phase-shift masks having the constitution as above are exposed to the short-wave light as above, the transparent layer therein will inevitably act to control and lower its transmittance, and, as a result, its transmittance will be lower than the necessary level for halftone phase-shift masks.
One method for solving the problem is to reduce the thickness of the high-transmission layer in the photomasks to be exposed to short-wave light—the layer corresponds to the transparent layer in the current photomasks to be exposed to ordinary, not shortened light. In this case, however, the phase shift to be attained by the high-transmission layer is not good.
When a conventional single-layered halftone phase-shift mask is exposed to such short-wave light, it is difficult to control both the transmittance and the film thickness of the mask to such a degree that the mask exposed to the light could produce the desired phase shift. For example, amount of the phase shift φ (rad) of light having passed through the phase shifter part of a single-layered halftone phase-shift mask is represented by the following formula (1):φ=2πd(n−1)/λin which n indicates the refractive index of the single-layered film to form the phase shifter part of the mask, d indicates the film thickness, and λ indicates the wavelength of the transmitted light.
Therefore, if the film thickness of the phase shifter part of the mask is reduced in order to prevent the transmittance of the mask from being reduced, the mask could not produce a satisfactory phase shift.
On the other hand, in a multi-layered phase-shift mask but not a single-layered one, the phase shift control and the transmittance control may be effected independently in the different constituent films, and therefore, the multi-layered phase-shift mask could have ideal transmission characteristics. However, the conventional film constitution as in the above-mentioned patent specifications could hardly realize the intended transmission characteristics effective in the wavelength range of F2 excimer laser.
For example, the film constitution of four layers of Si3N4 and TaN alternately laminated in that order, disclosed in U.S. Pat. No. 5,939,227; and the film constitution of AlN/MoNx, AlN/TiN, or RuO2/HfO2 alternately multi-laminated in that order, disclosed in International Patent Publication 511301/2000 could not have optical characteristics (transmittance, phase shift angle) effective in the wavelength range of F2 excimer laser.
Another problem with the multi-layered film constitution is that the reflectance control of the film, which is necessary for mask inspection and for increasing the accuracy in the actual exposure process, is not easy.
In addition, in the process of producing the multi-layered film constitution, the etchability of each layer must be taken into consideration. For this, an important matter is that the multi-layered film constitution facilitates as much as possible the etching process. In particular, it is important that the multi-layered film is so planned that the substrate is not etched in the dry-etching process and the intended phase difference is accurately controlled.